To solve this without adding expensive dual-port RAM, the ULA implements . When the ULA is actively drawing the visible part of the screen, it takes absolute priority over the memory bus. If the Z80 CPU tries to access the lower RAM at that exact moment, the ULA dynamically pauses the CPU by halting its clock signal or pulling the Z80's WAIT line low. The CPU is only allowed to proceed during the brief microsecond windows when the ULA is drawing the screen borders or processing a horizontal blanking interval. 4. Keyboard Matrix Scanning
A microcomputer requires a rhythmic heartbeat to sync its components. The ULA connects to an external crystal oscillator and divides that high-frequency clock down to yield a stable , which it directly feeds into the Z80 CPU. 3. The Memory Contention Conflict
ZX Spectrum ULA: How to Design a Microcomputer , written by , is considered the definitive technical resource for understanding the custom "heart" of the Sinclair ZX Spectrum . The book documents Smith’s extensive reverse-engineering project, where he stripped the ULA chip down to its transistors to reveal its hidden logic and design secrets. Core Technical Focus To solve this without adding expensive dual-port RAM,
Before we open the schematic, you must adopt the 1982 mindset. You are not Apple. You cannot use a dozen LS TTL chips. You have to sell this computer for under £100.
This reverse‑engineering work has been central to many community projects. For example, the ULA is often the most sought‑after component for repairs, and modern replicas have become vital resources for keeping vintage machines alive. The CPU is only allowed to proceed during
A ZX Spectrum Implementation on an FPGA With Modern Peripherals
Video Fetch & Shift Engine
Both the Z80 CPU and the ULA need access to the lower 16KB of RAM. Because video generation cannot be interrupted without tearing the screen display, the ULA takes absolute priority. If the Z80 attempts to read or write to lower RAM while the ULA is drawing the visible screen area, the ULA physically stops the Z80's clock signal. This is known as "contention," which slows down execution speeds slightly but protects video integrity. 3. Keyboard Matrix Scanning
The creation of the (Uncommitted Logic Array) is a classic "race against time" tale of British engineering, where brilliance met extreme budget constraints to change computing forever. The Problem: A Tangled Mess of Chips The ULA connects to an external crystal oscillator
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